Ray Diagrams (Cambridge (CIE) IGCSE Physics): Revision Note

Features of ray diagrams

• Ray diagrams can be described using the following terms:

  • principal axis

  • principal focus, or focal point

  • focal length

• The principal axis is defined as:

A line which passes through the centre of a lens

• The principal focus, or focal point, is defined as:

The point at which rays of light travelling parallel to the principal axis intersect the principal axis and converge or the point at which diverging rays appear to proceed

• Focal length is defined as:

The distance between the centre of the lens and the principal focus

Converging & diverging lenses

• A lens is a piece of equipment that forms an image by refracting light (opens in a new tab)

• There are two types of lens:

  • converging

  • diverging

Converging lenses

• In a converging lens, parallel rays of light are brought to a focus

  • This point is called the principal focus

• This lens is sometimes referred to as a convex lens

• The distance from the lens to the principal focus is called the focal length

  • This depends on how curved the lens is

  • The more curved the lens, the shorter the focal length

Converging lens diagram

Diverging lenses

• In a diverging lens, parallel rays of light are made to diverge (spread out) from a point

  • This lens is sometimes referred to as a concave lens

• The principal focus is now the point from which the rays appear to diverge from

Diverging lens diagram

Representing lenses

• In diagrams, the following symbols are often used to represent each type of lens:

Converging and diverging lens symbols